The present invention relates to a composite member comprising two or more different members bonded through a fitting structure, and a method for bonding the different members through a fitting structure. More particularly, it relates to a composite member comprising two or more different members which are bonded to each other with an adhesive composition controlled in its expansion coefficient through a fitting structure, and a method for producing the composite member by bonding two or more different members with an adhesive composition controlled in its expansion coefficient.
There is a method of using a solder for bonding, for example, a ceramic member and a metallic member. However, during cooling operation after bonding them at high temperatures, thermal stress is generated owing to the difference in thermal expansion coefficient between the different members or between the member and the solder used for bonding of these different members to cause separation at the bonded interface, or if one of the members is fragile, cracks occur in the vicinity of the bonded interface and sometimes the desired bonding strength or airtightness cannot be obtained. The products in which these defects are caused during the production steps must be disposed of as rejected products, and this results in increase of the production cost of these composite members. Moreover, if they are subjected to thermal cycles in use, the defects occur after use for a certain period to cause deterioration of reliability of the products.
When different members are bonded using a solder, a method is generally employed according to which the surface of the ceramic member to be bonded is plated with a metal such as Ni to ensure wetting between the ceramic member and the solder, then these members are disposed opposite to each other with a suitable space, the solder is poured into the space, and the members are bonded. There is another method according to which an additive such as Ti which can ensure wetting by forming a reactive layer of a nitride or an oxide on the surface of the ceramic member is added to the solder, without carrying out the plating with a metal. However, these methods are not preferred because they are not enough to reduce the thermal stress, and cracks are often formed on the side of the ceramic member which is fragile against thermal stress or separation is caused at the bonded part to adversely affect various performances such as bonding strength and airtightness required for composite member. As method for relaxing the thermal stress, generally employed are a method of using a metal low in thermal expansion coefficient as an intermediate material at the time of bonding and a method of using as an intermediate material a soft metal which is high in reactivity with ceramics and can relax the stress by plastic deformation. However, these techniques also suffer from the problems caused by difference in thermal expansion between the solder and the member, for example, low thermal cycle characteristics, and cannot necessarily be said to be high in general-purpose properties. Furthermore, there is a high-pressure solid phase bonding method which is now under development, but this method has unsolved problems for practical utilization and cannot provide a sufficient bonding strength.
On the other hand, as a composite solder, JP-A-6-126479 discloses a mixture of powders comprising materials higher in melting point than solder and used for bonding of a semiconductor chip and a substrate. However, this aims at solution of insufficient wetting of the conventional composite solder caused by the powder also present on the surface by filling a powder comprising materials higher in melting point than the solder only in the central portion of the solder per se, in other words, aims at increase of bonding strength at the bonded interface. This composite solder is not effective for decrease of thermal stress, and, hence, is not effective for solution of the problems caused by the thermal stress between the members to be bonded or between the member and the solder.
As a result of intensive research conducted by the inventors for attaining the above object, they have found an adhesive composition for bonding at least two different members which does not cause decrease of bonding strength in the vicinity of the bonded interface due to thermal stress generated during cooling operation after bonding at high temperatures and does not cause generation of cracks in the member weak against the thermal stress during the cooling operation with maintaining an appropriate bonding strength between the different members. A patent application No.10-52971 was filed on this subject.
That is, as a result of intensive research, the inventors have found that at least two different members can be bonded without causing decrease of bonding strength in the vicinity of the bonded interface due to thermal stress generated during cooling operation after bonding at high temperatures and without causing generation of cracks in the member weak against the thermal stress during the cooling operation with maintaining an appropriate bonding strength between the different members by using as a base a hard solder which is not restricted by the kind of the members or shape of the members and which leaves room for selection of bonding shape and by adding to the hard solder a fine particle material which lowers the thermal stress. Thus, the above patent application was filed.
As a method for bonding with the above composition, the patent application discloses a method which comprises disposing two or more different members differing in thermal stress so that they are opposed with a space enough to bond them and pouring the composition into the space or filling a given amount of ceramic or cermet fine particles and subsequently pouring a given amount of the hard solder, followed by cooling to produce a composite member. However, the composition cannot be used as it is for bonding the members not provided with a space therebetween enough to be able to pour a given amount of the hard solder.
In the case of bonding two or more different members which must be bonded through a fitting structure, especially, in the case of bonding the members with selecting a very narrow clearance of about 0.01-0.30 mm, various troubles often occur unless a solder is filled also on the side of the members as uniformly as possible. On the other hand, when the clearance is greater than the above upper limit, cracks sometimes occur due to the residual stress which is generated owing to non-uniform filling of the solder in the clearance and is contained in the solder. This will be more specifically explained. For example, when a member having a columnar dented portion and a member having a columnar protruded portion are fitted, if the solder to be uniformly filled in the columnar clearance formed by these members is one-sidedly filled on only one side because the clearance has a room to some extent, the balance of thermal shrinkage stress is lost in the process of cooling and solidifying the solder and the member having the columnar protruded portion is pulled to one direction, resulting in residual stress and cracks. Moreover, there are demands for dimensional accuracy and beautiful appearance of the composite member as a finished product. Thus, since it is substantially impossible to pour a given amount of a hard solder from a narrow clearance in the members having only such a narrow clearance, these different members cannot be bonded by the above method. The clearance here means a width of the space present between the wall surfaces of the different members in the fitting structure.